This invention relates generally to a digital data transmission system and specifically to the digital data transmission system incorporated in the Zenith/AT&T Digital Spectrum Compatible High Definition Television (DSC-HDTV) system recently tested by the Federal Communications Commission.
In the foregoing system, data is supplied in an input frame format comprising two and four level symbols arranged in repetitive data segments and then reformatted into a transmission frame. The two level symbols are more robust and are used to convey information that is of greater "importance". Two level symbols are better in terms of noise performance, but result in a reduced data rate. The input frame format comprises 12 data segments (DS) of two level symbols followed by 240 data segments of either two or four level symbols and nine data segments of four level symbols. The data is supplied in the form of two successive fields similar to, but otherwise unrelated to, the two successive fields in an NTSC system. A transmission bit map (TBM) is included in each field. The TBM consists of 15 bytes (120 bits), one for each of 120 data segment pairs, i.e. 120 groups of two contiguous data segments, or a total of 240 data segments. A TBM "1" bit indicates a segment pair of two level symbols and a " 0" bit indicates a segment pair of four level symbols.
The symbol data is reconfigured into a transmission frame format for enhancing the ability of the data to withstand expected interference from NTSC as co-channel signals and to optimize signal robustness, i.e. its ability to be processed with minimum errors and maximum data rate. In addition to the expected interference from NTSC co-channel signals, conventional "burst+ noise, which tends to affect lines or segments of data, is likely to be encountered. NTSC signal interference tends to affect vertically aligned data in the transmission frame due to its strong vertical edge video content. The invention is directed to overcoming the effects of such vertically oriented interference. A conventional Reed-Solomon (R-S) byte correction code is incorporated in the data segments of the transmitted signal. Since the R-S system corrects bytes of data rather than symbols, the invention seeks to pack a number of symbol errors into a single byte of data (when the signal is reconfigured in the receiver) to maximize the R-S effectiveness.